1. Field of the Invention
The present invention relates to a process for producing a shaped, sintered magnesia article having an enhanced resistance to hydration and an improved mechanical strength. More particularly, the present invention relates to a process for producing a shaped, sintered magnesia article consisting essentially of fine magnesia particles each uniformly coated with a forsterite coating layer and firmly bonded to each other, and thus having an enhanced resistance to hydration, a high density which is substantially similar to a theoretical density thereof, and an improved mechanical strength.
2. Description of the Related Art
It is well known that magnesia exhibits excellent physical properties, for example, a high melting point of about 2800.degree. C., an excellent electrical insulating property, and a high heat transmission property. It is also known that sintered high density magnesia articles exhibit a high transmission of infrared rays and visible lights. Due to the above-mentioned properties, attempts have been made to utilize the intered magnesia articles as high temperature insulating materials, high temperature illumination materials, or IC base plate materials.
However, in industrial usage, the sintered magnesia articles are practical only for restricted usages of, for example, crucibles and protecting tubes for thermocouples. This restriction is imposed because the sintered magnesia articles exhibit a poor resistance to hydration. That is, magnesia (magnesium oxide) is easily hydrated in the ambient air atmosphere, and thus is converted to magnesium hydroxide, which exhibits poorer physical properties than those of magnesia.
In order to enhance the resistance of magnesia to hydration, the following processes have been disclosed.
(1) A magnesia powder is mixed with an additional powder consisting of at least one member selected from calcium magnesium phosphate, magnesium phosphate, and silica powders by mechanical mixing under a dry condition or in a proper solvent, the resultant magnesia mixture is shaped into a desired form, and the resultant shaped magnesia mixture article is sintered at an elevated temperature.
However, this process (1) is disadvantageous in that the resultant shaped, sintered magnesia article exhibits an unsatisfactory resistance to hydration, a uniform mixing of the magnesia powder with the additional powder is very difficult, and the uneven mixture causes an undesirable irregular growth of the particles in the sintering step. Therefore, the resultant sintered article exhibits unsatisfactory thermal and mechanical properties.
(2) Japanese Unexamined Patent Publication (Kokai) No. 58-217,480 discloses another process in which a shaped magnesia article is preliminarily sintered at a temperature of from 900.degree. C. to 1100.degree. C., the resultant porous article is impregnated with a solution of an organic silicate compound, the organic silicate compound in the preliminarily sintered porous magnesia article is burnt to form a very fine silica particle layer on the magnesia article, and the resultant article is sintered at a high temperature of about 1400.degree. C., to allow the very fine silica particles to react with magnesia and thus to provide a forsterite coating layer on the magnesia article.
This process (2) is disadvantageous in that the procedures in the process are very complicated, the amount of the forsterite coating layers to be formed on the magnesia particles cannot be easily adjusted to a desired level, and the resultant forsterite coating layers are formed unevenly on the magnesia particles. Therefore, in this process (2), it is necessary to produce the forsterite coating layers in an excessively large amount, to completely coat the shaped magnesia article. This large amount of the forsterite coating layers causes an undesirably reduced heat transmission of the resultant shaped, sintered magnesia article.
(3) Japanese Unexamined Patent Publication (Kokai) No. 60-166,260 discloses a process in which a magnesia powder is mixed with a solution of an organic silicate compound, the resultant magnesia mixture is shaped into a desired form, and the shaped magnesia mixture article is sintered so that the organic silicate compound is converted to silica and the magnesia reacts with the silica to deposit the resultant forsterite on the surfaces of the individual magnesia particles.
In this process (3), after the magnesia powder is mixed with the organic silicate compound solution, the resultant dispersion must be filtered and dried, and the resultant dried mixture, which is in the form of grains, must be milled or pulverized. These steps complicate the process (3), and the resultant mixture powder is contaminated with inevitable impurities. Also, the organic substances in a large amount covering the individual magnesia particle surfaces hinder a close bonding of the magnesia particles with each other. Therefore, it is difficult to obtain a sintered magnesia article having a high density. Furthermore, in the process (3), it is difficult to adjust the amount of the forsterite coating layers on the magnesia particles to a desired level.